We have a fibreglass pool in the Canary Islands.
The pool water is desalinated sea water (as is all the island's tapwater).
The pool treatment originally was "Baquacil" products.

We have a heater installed (it acts like an air conditioner in reverse -extracting heat from the air, transferred via a heat exchanger to the water).
Pool cleanlines is now maintained through continuous salt electrolysis (around 20kilos of salt in pool), plus weekly cleaning, pH testing, alkalinity checking.

The pool is covered every night, and is (probably) in daily use as the house is rented fairly continuously.

We have a dark staining to the walls, steps and floor. The stain start originally in just one small area. The pool installer is suggesting a chlorine shock treatment, which he says will render the pool unusable for 4/5 days, then it should be OK.

As this is the fourth suggestion, but at least the one that reverts the problem to the installer, we are obviously somewhat concerned and unconvinced.

Thank you for your response Falcon Pools. The answer is no, we are not sure whether all the baquasil had been cleared from the pool - probably not as this was done in our absence. What should we be doing?